I have a working villager breeder and I'm building a trading hall. I need to hold onto my villagers until they're adults otherwise they can escape the hall. I was thinking of putting them in somekind of loop until they've grown up but I can't find a way of detecting the difference. Observers can't see babies or adults (at least, not when they're in minecarts), trip wires don't detect either, as they go past in a minecart. Is there a standard pattern for doing this?
I can't guarantee it will work on Bedrock - best test it in creative first - but Gnembon has developed an ingeniously simple and easy to build device that holds baby villagers until they grow up.
The idea is ingenuious in its simplicity. The short bubblevator (with the standard push-pull features for pulling mobs into the water column - a wall block, and a gap for water to flow in) ends with a glass block 'cap', so any babies will get trapped under the glass block. (don't worry, they won't drown - bubble column provides air.) The moment they grow, their feet remain on the same level, but their heads glitch into the glass block above - it stops blocking them, so the bubblevator is able to push them even higher. Then they are caught by the water stream above and pushed through the open fence gate into the collection area.